This invention relates to an apparatus for measuring the relative power consumed during a welding process and, in particular, covers an apparatus to be used as a transducer in connection with a Soudronic welder adapted to weld the longitudinal side seam of a thin metal can body. Soudronic welders for this type of application have a secondary transformer rating of 4 to 8 volts and 5000 amps. The welding is AC resistance type in the frequency range of about 50 to 500 Hz with each alternating waveform producing a power pulse. A traveling electrode being a copper wire is positioned between the surfaces to be welded and the electrode rolls connected to the output terminals of the secondary winding of the welding transformer. The copper wire is used between each of the electrodes and the metal surface to be welded and is moved continuously in order to prevent deterioration of the welding electrodes.
Can bodies are generally hollow cylindrical constructions which are formed along a longitudinal edge into a closed cylinder leaving both ends open. The meeting edges of the cylinder thus formed from a flat blank of material are overlapped for purposes of welding. The blanks are preferably fashioned from preprinted (lithography), tinplate or tin free steel chrome-type such as MRT3. Such material presents ranges from 65 to 112 pound plate weight per base box which represents a range of 0.007" to 0.0123" in thickness depending upon the application of the container to be formed from the tinplate and/or tin free steel chrome-type. A welded side seam is preferable to other forms of side seams such as a soldered can seam or a glued together joint. More particularly, in aerosol containers which must be capable of withstanding up to 200 pounds per square inch of internal pressure, a welded longitudinal side seam has a great many advantages. Similarly, in containers which are of a particular configuration which is too large to be drawn (as, for example, a two-piece container is), a welded side seam gives the requisite strength and simplifies the manufacture of such containers as they are too long or too large for drawing. In other applications it is important to have lithograph information on the exterior surface of the containers. Quality lithography cannot be applied at high-speed to a preformed drawn container so a container with a manufactured side seam is required.
Hall effect devices have been used in connection with a number of transducer applications some of which have been applied to welding machines see, for example, U.S. Pat. Nos. Noth 3,240,961; Hill 3,194,939; Barnhart et al 3,335,258 and Treppa et al 3,389,239. Each of the foregoing is designed to use a Hall device in combination with a welder for purposes of current determination. Similarly, the Hood U.S. Pat. No. 3,365,665 shows a Hall transducer which has been used in a system for measuring current flow in high voltage conductors e.g. power lines. Assignee of the present invention has a co-pending application on a Hall effect transducer, U.S. Ser. No. 093,855. These arrangements are not entirely responsive to the condition of the metal to be welded in that they primarily sense current flow and do not take into account the relative position of the welding electrodes. In the past welding monitors using voltage, current or Hall effect measuring transducers have been used to determine the condition of the power flow during welding. These techniques have been deficient in that they measure only one parameter which makes up the power available between the welding electrodes.
Other techniques that have been used as a means of monitoring weld quality do not possess the desired lack of sensitivity to outside effects and in most cases measure a parameter that does not totally characterize the quality of a weld. For example, monitoring welding electrode voltage only insures that a voltage is present that is sufficient to produce the necessary heating if all other factors are constant such as surface resistance and plate integrity. If either of these factors vary there will be no indication of it by monitoring the welding voltage.
As another example, monitoring welding current will yield information that insures that each attempt at weld nugget formation has sufficient current available to produce the required heating. However, should the plate weight vary, for example a 10% increase, there is no indication that welding current will change significantly since a 10% increase in thickness will result in an insignificant change in bulk material resistance. However, a 10% increase in thickness can have a significant effect on the rate of heat dissipation and the amount of metal which must be heated to an acceptable temperature. Without a corresponding change in welding current for a material thickness increase no detectable information is available on which to act.
Neither voltage nor current monitoring or the combination of the two will accurately account for the insidious effects of intermittent. variable and unpredicted shunt resistance paths. These can momentarily alter the current flowing through the desired weld zone and thereby effect the weld nugget quality without leaving a measurable trace. As a single measurable parameter the weld forging roll dynamic motion offers a method of singularly monitoring the effect of any or all weld parameter variations and to provide an indicator value with which to adjust the easily altered welding current parameter. Electrode force or voltage are other control parameters which could be adjusted. In short, monitoring the weld forging roll dynamics appears to be a good measurement tool for the purpose.
In a high-speed operation such as welding thin metal can bodies at several hundred per minute with an alternating current welder, the influences of input current and voltage as well as ambient temperature becomes significant when one is trying to measure small changes in the welder operating conditions. It is, therefore, the function of the circuit herein to completely compensate for the aforesaid conditions by providing an electrode motion responsive transducer which will be useful in monitoring the electrode forging action used to weld the side seam of a thin metal container and same will be set forth in the following summary.